This invention relates to zeolite bound zeolite catalyst which can be tailored to optimize its performance and the use of the zeolite bound zeolite catalyst in hydrocarbon conversion processes.
Crystalline microporous molecular sieves, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion processes. In addition, the crystalline microporous molecular sieves have been used as adsorbents and catalyst carriers for various types of hydrocarbon conversion processes, and other applications. These molecular sieves are ordered, porous, crystalline material having a definite crystalline structure as determined by x-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. The dimensions of these channels or pores are such as to allow for adsorption of molecules with certain dimensions while rejecting those of large dimensions. The interstitial spaces or channels formed by the crystalline network enable molecular sieves such as crystalline silicates, aluminosilicates, crystalline silicoalumino phosphates, and crystalline aluminophosphates, to be used as molecular sieves in separation processes and catalysts and catalyst supports in a wide variety of hydrocarbon conversion processes.
Within a pore of the crystalline molecular sieve, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, disproportionation, alkylation, and transalkylation of aromatics are governed by constraints imposed by the channel size of the molecular sieve. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the pores to react; while product selectivity occurs when some of the products can not leave the channels or do not subsequently react. Product distributions can also be altered by transition state selectivity in which certain reactions can not occur because the reaction transition state is too large to form within the pores. Selectivity can also result from configuration constraints on diffusion where the dimensions of the molecule approach that of the pore system. Non-selective reactions on the surface of the molecular sieve, such reactions on the surface acid sites of the molecular sieve, are generally not desirable as such reactions are not subject to the shape selective constraints imposed on those reactions occurring within the channels of the molecular sieve.
Zeolites are crystalline microporous molecular sieves that are comprised of a lattice silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term xe2x80x9czeolitesxe2x80x9d includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, titanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms xe2x80x9czeolitexe2x80x9d, xe2x80x9czeolitesxe2x80x9d and xe2x80x9czeolite materialxe2x80x9d, as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon, and aluminum such as gallosilicates, borosilicates, silicoaluminophosphates (SAPO) and aluminophosphates (ALPO). The term xe2x80x9caluminosilicate zeolitexe2x80x9d, as used herein, shall mean zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
In certain hydrocarbon conversion processes, it is sometimes desirable that the catalyst used in the process be tailored to maximize its performance in specific hydrocarbon conversion processes. For instance, it is sometimes desirable that the catalyst used in a hydrocarbon conversion process be a multifunctional catalyst, e.g., a trifunctional catalyst or a bifunctional catalyst. A bifunctional catalyst comprises two separate catalysts, e.g., two zeolites having different compositions or structure types, which induce separate reactions. The reaction products can be separate or the two catalysts can be used together such that the reaction product of one catalyst is transported to and reacts on a catalyst site of the second catalyst. Also, since one of the benefits of using a zeolite catalyst is that the catalyst is shape selective and non-selective reactions on the surface of the zeolite are usually not desirable, it is sometimes desirable that the catalyst used in a hydrocarbon conversion process have the capability of preventing or at least reducing unwanted reactions which may take place on the surface of the zeolite catalyst by selectively sieving molecules in the feedstream based on their size or shape to prevent undesirable molecules present in the feedstream from entering the catalytic phase of the zeolite catalyst and reacting with the catalyst. In addition, the performance of a zeolite catalyst can sometimes be maximized if the catalyst selectively sieves desired molecules based on their size or shape in order to prevent the molecules from exiting the catalyst phase of the catalyst.
Hydrocarbon conversion using catalysts containing two different zeolites have been proposed in the past. For example, U.S. Pat. No. 5,536,687 involves a hydrocracking process using a catalyst containing crystals of zeolite beta and zeolite Y that are bound by an amorphous binder material such as alumina.
Zeolite crystals have good adsorptive properties, but their practical applications are severely limited because it is difficult to operate fixed beds with zeolite powder. Therefore, prior to using the crystals in commercial processes, mechanical strength is conventionally conferred on the zeolite crystals by forming a zeolite aggregate such as a pill, sphere, or extrudate. The extrudate can be formed by extruding the zeolite crystals in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate. The binder materials used are resistant to the temperatures and other conditions, e.g., mechanical attrition, which occur in various hydrocarbon conversion processes. It is generally necessary that the zeolite be resistant to mechanical attrition, that is, the formation of fines which are small particles, e.g., particles having a size of less than 20 microns. Examples of suitable binders include amorphous materials such as alumina, silica, titania, and various types of clays.
Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when the bound zeolite is used in a catalytic conversion process, the performance of the catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the amorphous binder. For instance, since the binder is typically present in amounts of up to about 60 wt. % of the bound catalyst, the amorphous binder dilutes the adsorptive properties of the zeolite aggregate. In addition, since the bound zeolite is prepared by extruding or otherwise forming the zeolite with the amorphous binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite or otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to and from the pores of the zeolite which can reduce the effectiveness of the zeolite when used in hydrocarbon conversion processes and other applications. Furthermore, when a bound zeolite is used in catalytic conversion processes, the amorphous binder may affect the chemical reactions that are taking place within the zeolite and also may itself catalyze undesirable reactions which can result in the formation of undesirable products. Therefore, it is desirable that zeolite catalysts used in hydrocarbon conversion not include deleterious amounts of such binders.
The present invention provides a zeolite bound zeolite catalyst for use in hydrocarbon conversion processes which overcomes or at least mitigates the above described problems and can be tailored to optimize its performance.
The present invention is directed to a zeolite bound zeolite catalyst that can be tailored to optimize its performance in hydrocarbon conversion. The zeolite bound zeolite catalyst contains core crystals comprising first crystals of a first zeolite and optionally second crystals of a second zeolite having a composition or structure type that is different from said first zeolite and binder crystals containing third crystals of a third zeolite and optionally fourth crystals of a fourth zeolite having a composition or structure type that is different from said third zeolite. If the core crystals of the zeolite bound zeolite catalyst do not contain, in addition to the first crystals of the first zeolite, second crystals of said second zeolite, then the binder crystals will contain, in addition to the third crystals of the third zeolite, fourth crystals of fourth zeolite. The zeolite bound zeolite catalyst can contain both second crystals of a second zeolite and fourth crystals of a fourth zeolite. The structure type and/or composition of the zeolites are usually tailored to provide a zeolite bound zeolite catalyst having enhanced performance. For example, the zeolite bound zeolite catalyst can be tailored to be multifunctional and/or can be tailored to prevent undesirable molecules from entering or exiting the catalytic phase of the zeolite bound zeolite catalyst.
In another embodiment, the present invention provides a process for the conversion of hydrocarbon feeds using the zeolite bound zeolite catalyst. Examples of such processes include processes where catalyst acidity in combination with zeolite structure are important for reaction selectivity, e.g., catalytic cracking, alkylation, dealkylation, disproportionation, and transalkylation reactions. The process also finds particular application in hydrocarbon conversion processes in which carbon-containing compounds are changed to different carbon containing compounds. Examples of such processes include dehydrogenation, hydrocracking, isomerization, dewaxing, oligomerization, and reforming processes.